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Full text of "Text Book Of Mechanical Engineering"

Appendix V.                              973

E in gear and causing the tup to rise till M and j meet to form
the rivet head. The tappet v has thus reached the nut w and
raised the striking bar x together with the handle u, compelling
the tup to once more descend. But a stop rod Y passes upward
through N and M to an arranged height suitable for the rivet
length, so when the whole piece K M N strikes the buffer plates z,
the rivet is ejected by its meeting rod Y. The workman imme-
diately inserts another shank piece, and the tup ascends auto-
matically on account of tappet v meeting nut b and so depressing
the handle j.

^ Pp. 124. and 748. Case-hardening.—An excellent form of
animal charcoal is manufactured by Messrs. Palfreyman, of Liver-
pool, for case-hardening purposes. Pure white hard bone is
cleared of pith and grease, and then carbonised in closed retorts
at a uniform temperature. After removal, and when still hot, it
is charged with a pure hydro-carbon oil, which prevents the
accession of damp—a very great desideratum. The charcoal
thus obtained is said to have three times the carbonising power
of ordinary bone black: certainly much more than can be
obtained with natural bone or leather, where the preparation
has to be done in the boxes themselves; and the colour on the
work is better and more uniform. The firm publish a pamphlet
in which they make the following recommendations :—

I^et the boxes be sufficiently large to allow 2" space all
round the work for pieces 4" to 6" diameter, for which boxes
i2"x I2"x8" deep would probably be suitable; but for smaller
work up to f" screws say, the boxes may be 4" x 4" x 8" deep.
When laying the articles among the charcoal, press the latter
lightly into the crevices or corners to ensure contact, and fill up
the last 2" with old or waste black that has been well dried : then
•cover with a suitable lid, dilute with clay, so as to resist the
outlet of gas. The boxes being placed in the furnace, the heat
is raised gradually, kept at cherry red for 3 hours for small
articles, and at bright orange for 15 to 24 hours with large ones,
a depth or 'case1 of £" being produced after 18 hours of heat.
For steel, 257. more black is needed than for iron.

If the articles are to be hardened, as is most usually required,